1. Field of the Invention
The present invention relates to a laser irradiation apparatus and a method of manufacturing a semiconductor device, including a step of crystallizing a semiconductor film with laser or strong light using a laser apparatus of the invention. The semiconductor apparatus refers to all of devices that can function by using semiconductor properties, for example, an electro-optical device such as a liquid crystal display device, an electric apparatus including an electro-optical device as its component, a light emitting device, and a device which receives/transmits data wirelessly, such as a wireless chip, a wireless IC, an RFID chip, an IC chip, or an IC tag.
2. Description of the Related Art
A technology is widely studied whereby laser irradiation is performed on a semiconductor film formed over an insulating substrate, for the crystallization, the improvement of crystalline characteristics, and/or the activation of a dopant added to the semiconductor film. A silicon film may be often used as the semiconductor film.
A glass substrate is often used as the insulating substrate because it can be processed into a large substrate. Other than the glass substrate, a quartz substrate may be used. However, it is significantly difficult to process a quartz substrate into a large substrate. It is very advantageous to use a glass substrate; however, the melting point of a glass substrate is problematic which is lower compared to that of the quartz substrate. Since a relatively high temperature is required for crystallizing a semiconductor film, there is a problem of deformation of a glass substrate at a time of crystallization. Correspondingly, crystallization using laser irradiation of the semiconductor film was devised in order to overcome the problems. By using laser, it is possible to only increase the temperature of the semiconductor film without increasing the temperature of the glass substrate. Accordingly, laser is preferably used for crystallizing a semiconductor film formed on a material having a low melting point such as a glass substrate or a plastic substrate.
A semiconductor film can be crystallized and/or the crystalline characteristics can be improved by irradiating a semiconductor film with laser. However, during the treatment, the semiconductor film is melted by laser energy and then returns to a solid state again. When it returns to the solid state, the semiconductor film forms innumerable nuclei. Each of the nuclei grows mainly in a direction parallel with a surface of the semiconductor film to form crystal grains. During the growth of the crystal grains, the adjacent crystal grains collide with each other. Thus, very high projecting portions are formed on the semiconductor film. Especially when the semiconductor film is irradiated with laser in an atmosphere containing oxygen, for example, in the atmospheric air, the projecting portion grows significantly and may reach to substantially the same height as the thickness of the semiconductor film. In this way, projections and depressions are formed on the surface of the semiconductor film having been irradiated with laser. Especially when a top gate TFT is manufactured, the surface having the projections and depressions becomes an interface against a gate insulating film. Therefore, the projections and depressions cause variation in device characteristics and/or increase in off current values.
It is known that the growth of the projecting portion can be suppressed significantly if oxygen is removed from the atmosphere for crystallizing the semiconductor film by irradiation with laser. Therefore, laser irradiation for crystallization is often performed in an atmosphere free from oxygen, for example, in a nitrogen atmosphere or in a vacuum. Thus, the projections and depressions of the semiconductor film may be suppressed. However, in order to have the atmosphere of nitrogen or the vacuum for the laser annealing, a rigid vacuum chamber, a vacuum system and/or supply of a large amount of nitrogen are required, which increases the cost.
In order to achieve the objects, an invention was disclosed by which only an atmosphere in close proximity to a semiconductor film irradiated with laser is made to be an atmosphere without oxygen, for example, a nitrogen atmosphere, a rare gas atmosphere, or a hydrogen atmosphere without using a expensive vacuum system, thereby forming an atmosphere locally containing small amount of oxygen (Reference 1: Japanese published patent application No. 2003-017411)
In the above reference, very strong airflow is formed, and a gas not containing oxygen is blown on an area of the semiconductor film which is irradiated with laser light during the laser irradiation. Alternatively, a flat plate capable of blowing a gas not containing oxygen is provided in close proximity to the semiconductor film and the semiconductor film is irradiated with laser light through the plate. In order to reduce the oxygen level to the minimum in the vicinity of the area of the semiconductor film which is irradiated with laser light, the plate blows a gas, so that the plate may be floated in the air above the semiconductor film.
When laser irradiation is performed in the atmosphere without oxygen, the depressions and projections of the surface of the semiconductor film can be reduced. On the other hand, the characteristics of the semiconductor may be deteriorated. In order to solve the problem, a method of irradiating a semiconductor film with first laser light in an atmosphere containing oxygen for crystallization, then removing an oxide film which is formed by the irradiation with the first laser light, and irradiating the semiconductor film with second laser light after that in an atmosphere not containing oxygen thereby reducing depressions and projections of the surface of the semiconductor film, that is, planarizing the surface of the semiconductor film is used. Through those steps, the surface of the semiconductor film can be planarized without large decrease in a current value of a TFT. Accordingly, the problems, especially, increase in an OFF current value can be suppressed.
By using a technology of reducing a time required for crystallizing a semiconductor film having an amorphous structure by adding a minute amount of a metal element such as nickel, palladium, or lead, a semiconductor film can be obtained which has a crystalline structure with good characteristics by performing heat treatment in a nitrogen atmosphere at 550° C. for four hours, for example (Reference 2: Japanese Published Patent Application No. 7-183540). The technology can not only reduce the heating temperature required for the crystallization but also increase the uniformity of the crystal orientation in a single direction. Producing a TFT based on the semiconductor film with such a crystal structure can not only enhance the field-effect mobility but also decrease the subthreshold coefficient (S value). Thus, the electrical characteristics can be improved significantly. When laser irradiation is performed in addition to the heat treatment, the characteristics as a semiconductor film can be improved as compared to a case where one of either heat treatment or laser irradiation is performed for the crystallization. The irradiation may be performed as irradiation with the first laser light and an oxide film formed by the irradiation can be removed. Further, after that, irradiation with the second laser light can be performed. Notably, in order to obtain higher performance, conditions for the heat treatment and the laser irradiation must be optimized.